This invention relates to sensors suitable for locating objects positioned behind or within a volume of material, and in particular to a wall-stud sensor.
U.S. Pat. No. 4,464,622, issued Aug. 7, 1984, discloses an electronic sensor for locating objects, such as wall studs, positioned behind wall surfaces. That sensor detects the edges of studs by measuring changes in the capacitance of a wall due to the presence or absence of a stud as the sensor is moved along the wall surface.
That sensor includes capacitive plates, or elements, connected to circuits that detect changes in the capacitance of those elements. As the sensor is moved along a wall, any such capacitive changes are presumably due to changes in the dielectric constant of the wall; increased capacitance is generally associated with the presence of a wall stud.
While this sensor works well to locate the edges of studs within most walls, experience has shown that its performance could be improved in several respects. For example, the precision with which that sensor locates a given stud edge depends upon various extraneous factors that affect wall capacitance, such as the thickness of the wall covering material, the dielectric constant of the wall covering material and the stud, the ambient humidity, and the temperature of the electronics within the sensor. Variances in some of these factors are compensated for by calibrating the sensor prior to each use by placing the sensor against the wall covering material at a place behind which there is presumed to be no stud. However, such calibration does not completely compensate for such variances. For example, changes in the thickness of wall covering thickness change the perceived location of stud edges. Furthermore, the calibration may be erroneous if there is a stud adjacent or partially adjacent the sensor during calibration. Accordingly, there is a need for a sensor that is easier to calibrate and less sensitive to variations in extraneous parameters that affect wall capacitance, particularly various wall thicknesses.